1. Field of the Invention
The present invention relates to an organic electroluminescent display having a plurality of organic electroluminescent devices.
2. Description of the Related Art
Organic electroluminescent displays (hereinafter, also referred to simply as “organic EL displays”) are expected as alternatives to the currently prevailing liquid crystal displays, and are under development for practical application. In particular, active matrix type organic EL displays having thin film transistors (TFTs) as switching elements are regarded as the mainstream of next-generation flat displays.
In general, an organic EL display has organic EL devices in which electrons and holes are injected into a luminescent layer from an electron injecting electrode and a hole injecting electrode, respectively. The electrons and holes recombine with each other at the interface between the luminescent layer and a hole transporting layer, near the interface, or inside the luminescent layer, thereby bringing organic molecules into an excited state. These organic molecules return from the excited state to a ground state with fluorescence emission.
Here, the material of the luminescent layer can be selected to obtain luminescent elements for emitting luminescence in appropriate colors. Besides, such luminescent elements can be selected appropriately to achieve a color display. In general, luminescent elements for emitting luminescence in the three primary colors of light, or red, green, and blue, have been developed and put to use.
FIG. 1 is a plan view of an organic EL display having organic electroluminescent devices (hereinafter, also referred to simply as “organic EL devices”) for emitting luminescence in red, green, and blue from their respective pixels. FIG. 1 schematically shows pixel areas of the three colors mentioned above. Starting from the left, there are provided a red pixel Rpix having a red luminescent layer, a green pixel Gpix having a green luminescent layer, and a blue pixel Bpix having a blue luminescent layer.
Each single pixel is formed in an area surrounded by gate signal lines 51 and drain signal lines 52. A first TFT 130, a switching element, is formed near the upper left intersection of the signal lines. A second TFT for driving the organic EL device is formed near the center. Organic EL devices are formed discretely in areas where hole injecting electrodes 12 of indium tin oxide (ITO) are formed.
FIG. 2 schematically shows a typical sectional view of organic EL devices having red, green, and blue, three types of luminescent layers. Starting from the left, there are the areas of the red pixel Rpix, the green pixel Gpix, and the blue pixel Bpix shown in FIG. 1. A hole injecting electrode 12, an interposed layer 14, and a hole transporting layer 16 are formed on a glass substrate 10 in order. A red luminescent layer 22, a green luminescent layer 24, and a blue luminescent layer 26 for emitting luminescence in red, green, and blue, respectively, are then formed in their respective predetermined areas on the hole transporting layer 26 so as to adjoin one another.
Subsequently, an electron transporting layer 28, an electron injecting layer 30, and an electron injecting electrode 32 are formed in this order on the three types of luminescent layers in common. In general, an organic EL display is made by vacuum deposition in a multi-chamber type organic EL manufacture system having a plurality of formation chambers. In particular, in the step of forming the red luminescent layer 22, the green luminescent layer 24, and the blue luminescent layer 26, the luminescent layers of desired colors are selectively formed in succession by mask processing in the same formation chamber.
By the way, as compared to such optical devices as liquid crystals, organic EL devices are significantly prone to degradation ascribable to secular changes. One of the reasons is that the electron transporting layer and the luminescent layer are susceptible to impurities such as water molecules and oxygen molecules. More specifically, oxidation of the organic substances constituting the electron transporting layers and the luminescent layers, and occurrence of bulk crystals near the surfaces of the respective layers can hinder electrons and holes from moving. It follows that the organic EL devices are degraded to lose desired luminescence characteristics.